The present invention relates to in general to a method and system for cleaning and priming an extrusion head, and in particular to performing such cleaning and priming in a thorough and space efficient manner adaptable for use an automatic coating apparatus.
Extrusion coating is a known method of directly depositing process coatings onto substrates, wafers, flat panel displays, and similar objects (collectively xe2x80x9csubstratesxe2x80x9d) in the microelectronics and display technology industries. According to a typical prior art system, substrates are transported linearly beneath an extrusion coating head, and process fluids are precisely dispensed from a linear orifice in the extrusion head using a microprocessor-based electrohydraulic pumping system. One such system is described in U.S. Pat. No. 4,696,885 entitled xe2x80x9cMETHOD OF FORMING A LARGE SURFACE AREA INTEGRATED CIRCUITxe2x80x9d. Depending on the particular application, such process fluids include photoresist, polyimides, color filter materials and the like. Such extrusion coating techniques are well-suited for research and development activities as well as high volume production requirements.
Although known extrusion systems of this type provide significant advantages as compared to other liquid deposition techniques (such as spin coating), they often suffer from a similar problem, specifically, the inability of the coating head to establish a uniform coating at the leading edge of the substrate during certain applications. In these systems, each substrate is a discrete part unlike a web coating process, and the coating deposition is therefore started and stopped with each new substrate. With such part by part processing, a coating xe2x80x9cbeadxe2x80x9d must be re-formed between the extrusion head and each new substrate to thereby xe2x80x9cwetxe2x80x9d the surfaces. When this bead initially contacts the substrate, however, it may cause a xe2x80x9cperturbationxe2x80x9d for some measurable distance (e.g., 5-20 mm) from the leading edge of the coating. Sometimes a leading edge anomaly of this type dictates that the substrate be rejected completely, thus increasing material and process costs and decreasing process efficiency.
There have been attempts in the art to address the problem of establishing a uniform coating condition in a linear or so-called slot type extrusion coater, and systems of this type are illustrated in U.S. Pat. No. 4,938,994 entitled xe2x80x9cMETHOD AND APPARATUS FOR PATCH COATING PRINTED CIRCUIT BOARDSxe2x80x9d and U.S. Pat No. 5,183,508 titled xe2x80x9cAPPARATUS FOR PATCH COATING PRINTED CIRCUIT BOARDSxe2x80x9d. In these patents, a controlled volumetric flow rate of the liquid is delivered to a liquid containing chamber within the extrusion head and then through the applicator slot to create what is said to be a uniform volumetric flow rate of liquid exiting from each point along the slot. A displacement piston associated with the extrusion head generates a fluid pulse to control the formation of a connecting bead of the liquid coating prior to, at the same time as, or after the sending of the controlled volumetric flow rate of the liquid. This technique purports to apply a layer of the liquid with a precisely-controlled volume per unit area of the liquid to the substrate. Prior art machines also include a slot sealing unit that cleans the extrusion head slot between applications. The prior art systems however, do not effect a condition for the extrusion head at the beginning of a coating operation which mimics the head""s condition in the middle of a coating operation. Therefore the problem of a gradual drift towards a steady state coating rate remains, resulting in leading edge perturbations. Therefore, prior art techniques do not adequately address the problem of leading edge perturbations that may affect the uniformity of the coating.
In order to avoid dripping or smearing coating material which has gathered around the extrusion head after a coating operation, it is often necessary to clean the extrusion head before a new coating operation begins. In the prior art, cleaning of extrusion mechanisms is usually accomplished manually, potentially leading to inconsistent results and disruption and delay of the coating operations. The presence of residual coating material on the extrusion during a coating operation can result in unwanted deposition of coating material, or contaminants which have collected on the residual material, on the substrate and/or on part of the coating apparatus. Therefore, it is a problem in the art that manual cleaning operations are inconsistent and unreliable.
Therefore there is a need in the art for a mechanism to overcome the problem of leading edge anomalies arising during the slot type coating of substrates in a batch process.
There is a further need in the art for a mechanism which will effectively and consistently clean an extrusion head so as to prevent dripping coating material onto a substrate or other surface.
These and other objects, features and technical advantages are achieved by a system and method which provides for a cleaning and priming assembly enabling automatic cleaning and priming of an extrusion or dispensing head at selected times.
A preferred embodiment of the present invention comprises a station comprising a cleaning station and a priming station at which cleaning and priming operations can preferably be performed automatically upon bringing an extrusion head to said station.
In a preferred embodiment of the invention, one cleaning operation would comprise a forceful cleaning process in which particularly viscous or heavy fluid buildups, including dried coating material, could be cleaned from the exterior of an extrusion head, which buildups are not amenable to being cleaned by fluid alone. Preferably, this forceful cleaning operation is performed at a scrubbing station. The forceful cleaning operation preferably involves direct mechanical contact between elements of the cleaning station and the extrusion head. In a preferred embodiment, this mechanical contact is in the form of scrubbers which contact a sufficient distance along the exterior of the extrusion head to remove all material buildup.
Preferably, the scrubbers comprise a large number of bristles which contact the extrusion head thereby transferring coating material from the head to the bristles. Alternatively, the scrubbers could comprise a surface comprising cloth, sponge or other suitable material which contacts the head and removes material through a combination of absorption and wiping contact.
The forceful cleaning mechanism, whether bristles or other device, preferably contacts the head in conjunction with a fluid spray or rinse to assist in transferring coating material away from the exterior of the head. Preferably, fluid would be kept in a reservoir and be continuously pumped toward the point of contact between the bristles or other contact device and the extrusion head. A constant pool of fluid could be used which would preferably be replaced at selected intervals. In a preferred embodiment, the fluid to be used in conjunction with the scrubbers is a strong solvent consistent with the expectation that this cleaning operation is directed toward cleanup of viscous material. The use of a solvent aids in the cleaning process by helping break down the coating material while it is still on the head surface and in keeping the scrubbers clean by dissolving coating material which has been transferred onto the scrubbers. Alternatively, a chemically inert or other fluid could be employed to provide lubrication for the contact of the cleaning mechanism with the head as well as to carry away coating material which has been removed.
Preferably, relative motion would be implemented between the bristles or other contact device and the head in order to achieve the most thorough possible scrubbing coverage of the surface to be cleaned of excess material. In a preferred embodiment, at least one cylindrical scrubber would rotate about an axis parallel to the head, against the surface of the head, thereby providing the desired relative motion within a compact space. The axis of this cylindrical scrubber is preferably fixed thereby aiding in providing a minimal footprint, but could be mobile, linearly or angularly, under spring loading or under some form of controlled motion so as to reach a greater surface area on the exterior of the head or to allow adjustment in the cleaning pressure exerted thereby.
Alternatively, the desired relative motion between the scrubbers and the head could be achieved through linear motion of either the head or the scrubbers or both. Further, the scrubbers could experience a combination of linear motion, and rotational motion about an axis perpendicular to the head thereby providing gyration, and enabling more through coverage of the area to be cleaned. Preferably the scrubbers comprise mechanical compliance permitting some amount of linear motion of the scrubbers in the direction of the head without damaging the head.
In a preferred embodiment, material is removed from rotating scrubbers which remove material from the extrusion head by disposing blades or other sturdy surfaces in contact with the scrubber surfaces which act to remove coating material therefrom. Preferably the blades or other surfaces are disposed on the lower side of scrubbers so as not to interfere with the extrusion head, but may be located anywhere within the reach of the scrubbers. The blades are preferably metallic but may be composed of any resilient and sturdy material.
In a preferred embodiment, a rinsing station is deployed as a part of the cleaning station which may be used as the sole cleaning station for the extrusion head, accordingly the above mentioned scrubbing station may be omitted in such an embodiment if desired, or may be used as a sub-station at which more refined cleaning of the head takes place after the head has been cleaned at a scrubbing station. Fluid for the rinsing station is preferably stored in a reservoir which is preferably distinct from the reservoir for the scrubbing station. Alternatively, fluid for both the rinsing and scrubbing stations could share the same reservoir.
Fluid at the rinsing station is preferably intended to remove either non-viscous coating material from the head left over from the coating operation with no scrubbing operation having been performed. Alternatively, fluid at the rinsing station can remove any solvents used at the scrubbing station in addition to any residual coating material left on the head after a scrubbing operation has been performed.
Fluid employed at the rinsing station is preferably self drying thus obviating the need for any further treatment of the extrusion head prior to either beginning coating or initiating priming of the extrusion head. Alternatively, solvent which is not self-drying could be used and means for drying could be employed to ensure that the rinsing fluid is completely removed from the extrusion head. Such means for drying the rinsing solvent off the extrusion head include, but are not limited to generating rapid air flow past the head, and generating heat in the vicinity of the head. However, it should be appreciated that the use of such air flow should be carefully controlled so as not to introduce gas bubbles into the extrusion manifold which may cause coating irregularities.
In a preferred embodiment, fluid for the rinsing station is stored in a reservoir below a station with a xe2x80x9cVxe2x80x9d shaped groove or similarly shaped cross section into which an extrusion head is brought into proximity. Rinsing fluid or solvent is then pumped from the reservoir through a fluid distribution structure so as to ensure complete and uniform rinsing fluid coverage of the extrusion head surface to be cleaned. Preferably, the fluid is pumped from the reservoir up through vertically oriented holes in the rinsing station material, then fed into a narrow slot generating a uniform curtain of fluid, thereby providing total coverage of the surface to be cleaned. Other possible geometries include, but are not limited to using a single vertical slot along the length of the rinsing station in communication with a second slot which directs the fluid toward the cleaning surface, a single slot beginning at the fluid reservoir initially directed vertically up from the reservoir, but appropriately bent at an appropriate stage so as to direct fluid toward the surface to be cleaned. In an alternative embodiment, the fluid reservoir could be located at the same vertical level as the surface to be cleaned thereby permitting a single straight slot to lead pressurized fluid directly from the reservoir to the surface to be cleaned. It is noted that a variety of possible fluid flow geometries are available which do not depart from the inventive mechanism embodied herein.
In another preferred embodiment, the means for directing rinsing fluid toward the surface to be cleaned comprises deployment of a porous or sintered material which when subject to appropriate pressure will supply a curtain of fluid flow toward the extrusion head surface, thereby providing universal rinsing fluid coverage of the surface to be cleaned.
In another preferred embodiment, a plurality of fluid sprayers, which may optionally be movable, in communication with the fluid reservoir and directed toward the extrusion head surface are deployed such that complete fluid coverage of the surface to be cleaned is achieved. Deployment of the sprayers may be such as to result in either overlapping or non-overlapping spray patterns onto the surface to be cleaned as long as the fluid contacts the entirety of the surface to be cleaned.
In an alternative embodiment, the rinsing station into which the extrusion head is brought into proximity, may have a number of possible shapes including but not limited to a half-circle, square, rectangular, oval and the like.
In an alternative preferred embodiment, the scrubbing and rinsing operations could be deployed at a single station employing either single or separate fluid reservoirs. The scrubbers and rinsing spray mechanisms would be disposed so as not to interfere with each other although each is disposed in a stationary manner. Alternatively, the scrubbers and/or rinsing fluid spray mechanisms could be movable so as to both be able to access the extrusion head without interfering with the other during operation.
In a preferred embodiment of the invention, a priming station is deployed, preferably in proximity to the cleaning station, for initiating a consistent coating bead, or steady state flow condition extrusion head. The extrusion head is preferably but not necessarily cleaned at one or more cleaning stations before being brought to the priming station. An extrusion head is best suited to begin a coating operation with a full and consistent coating bead and without any extraneous coating material present on the exterior of the head. Alternatively, the extrusion or coating head may be brought directly to the priming station without first going to the cleaning stations, particularly if the head is starting its first coating operation within a particular production run. The extrusion head may otherwise proceed directly to the priming station without first going to the cleaning stations if the head is sufficiently clean as to not require cleaning operations at any cleaning station. The number of coating operations between cleaning operations may vary depending on various factors including the coating material used, the head gap (distance between the extrusion head and a substrate being coated) employed, and the size of the substrate.
In a preferred embodiment, the priming station comprises a rotating cylindrical roller immersed in a bath of solvent, in contact with a brush cleaning the roller of coating material transferred from the coating head. The extrusion head is brought into close proximity to the roller, which is preferably metal or other material sufficient to simulate the surface of the substrate to be coated, and begins to extrude coating fluid onto the roller. The initial release of fluid from the head may be inconsistent for a certain period of time. Without a priming process, such inconsistencies would lead to leading edge anomalies on a surface being coated. The extrusion head remains over the roller extruding fluid in the same manner as it would when coating until either a sufficient amount of time elapses or a sufficient amount of coating fluid has been extruded over the priming roller to ensure that the coating bead is now ready for the actual coating operation. Another option for determining completion of the priming process involves sensing the existence of a full coating bead using sensing mechanisms including but not limited to a vision system disposed to view the coating bead, or contact or pressure sensors within the extrusion head. Another mechanism for determining completion of the priming process involves sensing the coating on the roller or other priming surface employing a vision system or contact sensors.
An extrusion head is properly primed when the coating bead is full and uniform across the extrusion geometry of the extrusion head. Where the extrusion head comprises an extrusion slot, priming is complete when a constant volumetric flow rate is reached across the full length of the slot, and the coating bead is full and uniform across the cross sectional area of the slot.
The priming roller receives the coating material deposited on it, effectively simulating travel of the extrusion head over a distance equal to the linear distance corresponding to the motion of the outside surface of the roller. The roller continuously rotates thereby rapidly exposing the deposited coating material to the solvent which acts to dissolve the coating material. A roller cleaning instrument, preferably a brush or blade, in contact with the roller, preferably underneath the roller so as to avoid interference with extrusion head, acts to remove still more of the coating material from the priming roller. The effect of the solvent bath and the brush, or other material removal device, is to clean the roller thoroughly enough that the portion of the roller surface emerging from the bath is ready to receive more coating material from the extrusion head. This process occurs continuously until the coating bead on the extrusion head is ready for the actual coating operation.
In an alternative embodiment, the required relative motion between a priming surface and the extrusion head could be achieved by various means other than a rotating circular roller including but not limited to a band wrapped around two or more rollers which passes under the extrusion head simulating travel of the extrusion head over a distance of material to be coated. Such a band would operate in much the same way as a treadmill with a parallel set of rollers turning in the same direction driving the band under the extrusion head, into a fluid bath, then preferably into contact with a band cleaning instrument, preferably a brush. The band would continuously revolve thus accomplishing much the same function as the roller although requiring a larger footprint.
Alternatively, a surface for coating fluid deposition could be linearly moved back and forth under the extrusion head with each portion of the surface having coating material thereon being cleaned thereof before being moved back under the extrusion head to receive more coating fluid. Yet another alternative embodiment involves having the extrusion head move with respect to a mostly stationary priming surface wherein portions of the priming surface which have had coating fluid deposited thereon would be cleaned as quickly as possible after such fluid is deposited, thereby preparing this portion of the priming surface for the next pass of the extrusion head.
In a preferred embodiment the fluid used in the priming station is a strong solvent preferably capable of completely dissolving the coating fluid deposited on the priming surface employed for a number of such priming operations. After a preselected number of coating operations, the used solvent in the priming station would be replaced with a fresh supply. Alternatively, the fluid could be chemically inert and merely mechanically aid the brush or other priming surface cleaning device in removing coating fluid from the priming surface. In this case, the fluid used would preferably be lighter than the coating fluid so that the coating fluid would settle at the bottom of a common fluid chamber. Alternatively, the coating fluid could be filtered out of the priming station fluid by appropriate means and stored for later removal from the fluid station and possible recovery or recycling.
In a preferred embodiment, a brush is brought into contact with the priming surface to help remove coating fluid from said surface. A number of other priming surface cleaning mechanisms could be disposed close to, or in contact with, the priming surface including but not limited to a sponge preferably in motion either linearly or angularly with respect to said priming surface, and a sharp edge located in close proximity to the priming surface for scraping away any coating fluid remaining on the priming surface.
Determination as to when to stop the priming process may be made based on a number of conditions including but not limited to the amount of time spent dispensing fluid by the extrusion head over the priming surface, the amount of fluid expended by the extrusion head, and determination by sensor means that the coating bead is ready for an actual coating operation.
In an alternative preferred embodiment, the fluid priming operation can be conducted within the rinsing station or the scrubbing station. Priming the head in this manner requires that the coating fluid be extruded for a preselected period of time, or that a preselected volume of fluid be extruded such that a steady state volumetric flow rate and reliable consistent bead is achieved. In this embodiment, the fluid is extruded into a cleaning station, either the rinser or the scrubber, which absorbs the extruded coating fluid into its solvent pool. The solvent solution is preferably chemically adapted to completely dissolve the extruded coating material. Alternatively, the extruded coating fluid can be segregated from the rest of the fluid in the pool by filtering means or through the ability to separate fluids of different densities.
Therefore, it is a technical advantage of the present invention that an extrusion head can be automatically cleaned by bringing the head into proximity of one or more cleaning stations which occupy a minimal footprint within, or in proximity to, a coating apparatus.
It is a further technical advantage of the present invention that different cleaning operations may be employed depending upon such factors as the viscosity and chemical composition of the fluid to be removed.
It is a still further technical advantage of the present invention to provide a priming mechanism to ensure that a steady state flow condition and proper coating bead exist at the extrusion head before a coating operation is undertaken.
It is a still further technical advantage of the present invention that properly priming the bead at the extrusion head can prevent leading edge anomalies when coating a substrate.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.